Diffusion coating

ABSTRACT

Diffusion coating of superalloys with aluminum is effectively prevented at desired locations by masking with nickel or cobalt aluminide having less than one atom of aluminum for each atom of nickel or cobalt, and packed against those locations. Chromium, silicon, iron or any combination of them can be coated along with the aluminum as by alloying with the aluminum used to provide the coating, and particularly effective particle size of such alloys is about 1 micron. Such sizes are produced by simultaneous magnesothermic reduction. Molybdenum and tungsten silicide particles can also be produced in this small size by such reduction and make effective diffusion coating materials. Diffusion coating can also be confined to restricted portions of work pieces without the above masking.

[ Apr. 2, 1974 DIFFUSION COATING [75] Inventor: Alfonso L. Baldi, Drexel Hill, Pa.

[73] Assignee: Alloy Surfaces Co., Inc.,

Wilmington, Del.

22 Filed: Jan. 20, 1972 [21] Appl. No.: 219,514

Related US. Application Data [63] Continuation of Ser. No. 837,811, June 30, 1969,

Primary ExaminerAlfred L. Leavitt Assistant Examiner-J. W. Massie [57] ABSTRACT Diffusion coating of superalloys with aluminum is effectively prevented at desired locations by masking with nickel or cobalt aluminide having less than one atom of aluminum for each atom of nickel or cobalt, and packed against those locations. Chromium, silicon, iron or any combination of them can be coated along with the aluminum as by alloying with the aluminum used to provide the coating, and particularly effective particle size of such alloys is about 1 micron. Such sizes are produced by simultaneous magnesothermic reduction. Molybdenum and tungsten silicide particles can also be produced in this small size by such reduction and make effective diffusion coating materials. Diffusion coating can also be confined to restricted portions of work pieces without the above masking.

17 Claims, 4 Drawing Figures nirFesioN COATING The present application is in part a continuation of application Ser. No. 837,8 ll filed June 30, 1969 now abandoned.

The present invention relates to the diffusion coating of metals, and particularly to introduce aluminum into the surface of high temperature superalloys. Such coating is described for example in US. Pat. Nos. 2,875,090 granted Feb. 24, 1959 and 3,226,207 granted Dec. 28, 1965, as well as in French Pat. No. 1,490,744 granted June 26, 1967, and other metals such as chromium and/or silicon and/or iron may be diffused into the surfaces along with the aluminum.

Diffusion coating of high temperature superalloys is carried out at very high temperatures, around 1,800F, and in a closed container that need not be sealed. Under these conditions the coating generally extends to all metal surfaces within the container even including the container walls. In many cases it is important to keep the coating from certain portions of the work pieces treated. Jet engine turbine blades for example should have roots that are not coated if their mounting dimensions are to be kept within tolerance. Also welded areas can be adversely affected by diffusion coating and are advantageously masked.

Shielding the root from the diffusion while exposing the balance of the blade to receive the coating is a difficult task particularly when a quantity of the blades is treated simultaneously in the same container. The atmosphere within the container becomes a very penetrating coating medium that tends to pervade everything in the container. Shielding or masking materials may be swamped and rendered ineffective, or if very active tend to cause undesired changes in the surface of the substrate.

Among the objects of the present invention is the provision of novel techniques for confining the foregoing diffusion of aluminum and of any other metal to desired locations. Additional objects of the present invention include improved materials and processes for diffusion coating.

These as well as other objects and advantages of the present invention will be more fully understood from the following description of several of its exemplifications, reference being made to the accompanying drawings in which:

H6. 1 is a vertical sectional view of a diffusion apparatus packed in accordance with the present invention;

H65. 2 and 3 are vertical and horizontal sectional views respectively of a modified diffusion arrangement pursuant to the present invention; and

FIG. 4 is a fragmentary view showing a further modified diffusion arrangement exemplifying the present invention.

According to the present invention cobalt and nickel aluminides containing less than one atom of aluminum for each atom of cobalt or nickel provide very effective masking of aluminum diffusion, as well as of the diffusion of other metals, into superalloys. The diffusion of the aluminum, whether or not masking is used, is also simplified by the use of a cementatlon pack in which the aluminum is in the form of particles of aluminumchromium, aluminum-silicon, aluminum-chromiumsilicon or aluminum-chromium-iron alloy about 1 micron in size. Such particles are conveniently produced by magnesothermically reducing a mixture of reducible compounds of aluminum and of the other metals of the alloy. The oxides of the various metals are particularly suitable for magnesothermic reduction.'

The figures of the drawing illustrate masking arrangements pursuant to the present invention. In FIG. l an outer metal retort 10 having a bolt-on cover 12 which can be equipped with a water-circulating jacket 14 and an O-ring seal 16, receives an inner metal retort 20 that is covered by a loosely fitting lid 22. The inner retort is almost completely filled with the diffusion coating materials including coating pack and work pieces to be coated, shown as jet engine blades 30 held in two rows in individual carriers 34. Each carrier is an elongated metal trough having a longitudinal slot 36 in its upper wall. A row of the blades is fitted into the slot with the blade roots 32 below, and the remainder of each blade above the slot. The trough can be readily fashioned from sheet metal bent to form a slot of the correct width for the particular blades to be treated.

The blade roots are embedded in a masking composition 40 which is a powdered mixture containing one or more of the abovementioned aluminides, and can be poured into the troughs from their open ends after the blades are all mounted. The open trough ends can then be closed as by bending the trough bottom or sidewalls. The closing need not be a complete sealing of the ends so long as it keeps the masking composition in place during the loading of the inner retort. The individual blades in a trough are preferably spaced from each other and there is no need to isolate the masking com position from contact with the surrounding coating ma terial 50 in these spaces. If desired a separator such as a metal sheet can be interposed between the masking composition and the coating material where they would otherwise come in contact with each other. This helps keep the two compositions separate when the retort is emptied so that each can be more readily reused. The separating sheet can be plain carbon steel or a type 400 stainless steel where the atmosphere in the coating retort is high in hydrogen or other reducing agents. Should that atmosphere lack sufficient reducing ability then it is preferred to have the interposed sheet of the same composition as the work pieces being coated.

The retort 20 can be loaded by first pouring in a layer of diffusion coating composition 50, then inserting a layer of one or more previously prepared and filled troughs 34, then pouring in additional coating composition 50 to cover the trough layer and its contents of blades, following which another layer of troughs can be inserted, and the steps repeated until the retort is filled to the appropriate level. The lid 22 is then applied either as a loose fit or it can be sealed, as by being welded on in which case it is desirable to leave a little space at the top of the inner retort so that particles of the coating composition do not interfere with the welding. Where the inner retort is sealed with a welded-on cover, it is desirable to provide a conduit through the cover or wall of the retort through which the internal pressure can be monitored and controlled.

The inner retort of FlG. l is shown loosely covered and inserted in the outer retort, a lifting ring 24 welded to the inner surface of the inner retort providing a convenient lifting arrangement for this purpose. The inner retort is shown as not filling the outer retort, gas introduction and discharge tubes 61, 62 being arranged in the space thus provided. Thermo-couples such as the one shown at 18 in a thermal well, as well as others in internally projecting pockets can also be fitted to appropriate locations in or on the inner retort and provided with leads extending through cover 12 to convenient reading and recording instruments. A supporting spider 19 can be placed'on the floor of the outer retort to receive the bottom of the inner retort.

The retort assembly, with cover 12 tightly secured, is now lowered into a furnace where it is heated to coating temperature, generally between about l,600 and 2,300F, and kept for the desired coating time. A blanketing gas such as hydrogen is flushed through the tubes 61, 62 and burned as it emerges.

The retorts and the troughs should be of material that withstands the furnace temperatures. Ordinary mild or low carbon steel is adequate for the inner retort and troughs. The outer retort has its exterior exposed to air during the coating operation and should therefore be of oxidation-resistant metal such as lnconel 600 and Incoloy 800, or the like.

In FIGS. 2 and 3 there is illustrated a selective coating arrangement in which only a small portion of a jet engine vane 70 is diffusion coated. Vane 70 is hollow and has cooling passageways 72 at its trailing edge as well as a welded-in cooling tube 76 with narrow discharge perforations 78 along its length. The trailing edge is quite thin and needs coating because rough handling has caused the spalling off of an originally applied coating. Because of the narrow size of the air outlet perforations 78 they cannot be exposed to coating conditions without having the coating deposit on the mouths of the perforations and unduly restrict them. To avoid having to remove and later replace the cooling tube, recoating can be effected in a localized area in the manner illustrated.

For this purpose one or more vanes 70 can be fitted in an inner retort 80 where each is embedded in a pack 84 that can merely be an inert supporting material such as powdered alumina. Before embedding, the trailing edge 74 of each vane is inserted in a slot 88 ofa tubular holder 86 and the holder filled with a diffusion coating composition 50. The holder 86 is shaped to extend just beyond the outer openings of the cooling passageways 72. It is not necessary to have the lips of the slot 88 engaging the vane, and a clearance of as much as 1/16 to A inch can be provided at each lip, so long as the powdered contents of the holder so not spill out. The thickness of the coating mixture contacting the vane surface should be at least about inch within about A inch of all surfaces to be coated, to assure the presence of sufficient coating material.

HO. 4 illustrates the partial coating of turbine wheels 100 having a number of air-foil cups 102 projecting from a body portion 104 provided with a central hub 106. The only parts exposed in use to the hot impelling gases are the air-foil cups 102 and the radially outermost surface of the body portion 104 between the cups, and these are the only parts diffusion coated. This avoids changing the dimensions of the remaining surfaces as well as changes in strength of those highly stressed portions. Such restricted coating is effected by stacking the wheels between masking rings 110 that have the same outer diameter as the body portions 104 of the wheels and hold masking composition. The stacking is readily effected by first placing a wheel with its axis vertical on a support, stacking a ring 110 over it, filling the ring with masking composition, covering the filled ring with a plate 112, clamping the assembly together, inverting it, and placing the inverted assembly in a retort containing a supporting layer of coating mixture, removing the clamps, placing a second ring over the top of the inverted wheel, filling the second ring with masking composition, and stacking more wheels and rings on top of the second ring. The topmost ring can be covered by another plate 112 although this is not essential, and the retort filled with coating composition.

The following examples illustrate the invention:

EXAMPLE I Using the arrangement of FIG. 1, jet turbine blades ofBl900 alloy having an overall length of about 4 inches are loaded in troughs 34, 30 to a trough and six layers of such troughs loaded in a diffusion retort. The B1900 alloy has by weight 8% Cr, 10% Co, 1% Ti, 6% Al, 6% M0, 4.3% Ta, 0.15% B and 0.01% Zr, the balance being nickel. The blade roots are embedded in a mixture of equal parts by weight of minus 325 mesh calcined alumina and about the same size powdered nickel-aluminum alloy formed by heating for 8 hours at 1,800F under hydrogen, a mixture of 2 gram-atoms of nickel and l gram-atom of aluminum. The heated product is a brittle alloy that corresponds to the nickel aluminide Ni Al.

The diffusion coating pack 50 is a pre-fired mixture of 1 micron size chromium powder, minus 325 mesh (about 17 micron average) aluminum powder, minus 325 mesh calcined alumina, and ammonium chloride as described in Example 1 of French Pat. No. 1,490,744. The retort assembly is heated to 1,950F in a gas-fired furnace and held there for six hours while a stream of hydrogen is passed through the space between the retorts and burnt as it emerges. Water is also circulated through the outer cover to keep it from getting too hot.

After the heat is completed the retort assembly is cooled down, disassembled, the coating pack 50 drawn off by suction, and the blade-carrying troughs lifted out as they are exposed. The blades show very good coatings about 0.003 inch thick on all parts that have been in contact with the coating pack 50, the coating tapering off to nothing within about inch of where the roots have rested against the trough surface. The roots show no dimensional, metallurgical or strength changes and pass the same fatigue tests as the untreated blade.

Substituting an unalloyed nickel and aluminum mixture for the nickel aluminide of the masking composition renders the masking ineffective to completely prevent diffusion coating of the blade roots. The substitution of nickel alone for the alloyed material maintains the masking effectiveness but weakens the blade roots, apparently due to excessive depletion of chromium and/or other ingredients from their skin during the coating treatment.

Masking results similar to that produced by Ni Al are also produced by nickel aluminides ranging from Ni Al to those having only slightly less aluminum than NiAl. With nickel contents larger than Ni Al the weakening of the masked portions becomes significant and treatment with such alloys is not desirable. On the other hand with Ni Al the masking does not keep the coating from extending V4 inch or more beyond the masked edge of the coating. The preferred range is from Ni Al to Ni,Al and best results are produced with Ni Al.

EXAM PLE ll Jet nozzle guide vanes 3 inches long and similar to those illustrated in F168. 2 and 3 but without the cooling tube 76, were diffusion coated with the same coating mixture used in Example I. These vanes were of Wi-52 alloy containing by weight 21% Cr QfQZb C, 1.8% Fe, ll% W 2 Eb plus Ta, the balance being cobalt. The interiors of the vanes were filled with the coating mixture but that mixture is kept from contacting the mouth of the opening leading to the interior of the vane. That opening is in a buttress portion of the vane and its mouth is used as a site for welding on a cooling tube. The edge wall of the mouth is accordingly masked with a layer of masking composition poured over the coating mixture filling the vanes interior. A cap can also be used below and/or above the layer of masking composition, but is not essential unless the masking layer is not level and needs supporting to stay in place. The masking composition used was a mixture of equal parts by weight minus 325 mesh alumina, and s i'atwray iia'cashed cobalt aluriiifiiimalloy made by heating powered cobalt and aluminum for 6 hours at 1,850F under hydrogen in the proportion of 3 gram-atoms of cobalt to 1 gram-atom of aluminum. The outer faces of the vanes buttresses need not be coated so that the masking layer can be thicker than needed to protect the mouth of the opening into the vanes interior. The coating was applied by a hour heat at l,980F, developing a case depth of 0.002 inch. After removing the coated vanes from the inner retort, freeing them from the coating and masking powder, and washing, the masked areas were bright and silvery and free of coating, the remainder being bright bronze-colored.

The masking packs of Examples l and II can be reused although it is preferred to separate and discard the portion of the pack that was in contact with the masked surface. About a A; inch depth of such portion can be so discarded.

The coating packs can also be reused, preferably after adding to them a small amount of fresh chromium and aluminum or pre-fired chromium-aluminum mix ture to replace that consumed by the coating operation. Some of the ammonium chloride is also lost during the coating, and this loss is desirably made up before reuse. instead of using a separate firing step to prepare the make-up chromium-aluminum mixture, a make-up quantity of an unfired chromium-aluminum mixture can be added to the very top of the coating pack so that during the coating step it is at least about an inch from the nearest surface to be coated. The high temperature of the coating operation will serve to pre-fire the unfired portion so that it can be incorporated in the body of the coating pack when the pack is reused. During a coating step, consumption of aluminum is on a weight basis two to five times as much as the consumption of chromium so that it is desirable to have the make-up quantities of these metals in the same proportions.

The separate step of pro-firing the chromium and aluminum mixture can be avoided by directly preparing such a mixture in finely divided form. To this end the magnesothermic reduction of chromium compounds such as Cr O as described in French Pat. No. 1,123,326 and its Addition Pat. No. 70,936, can be modified by combining an appropriate quantity of alu mina with the chromium compound, and such combination mixed and subjected to the magneso-thermic reduction as described in those patents. This simulta neous reduction takes place at about the same temperatures and times as is shown for the reduction of the chromium compound alone and with the same equipment, producing a chromium-aluminum alloy having a particle size of about 1 micron. Residual magnesium as well as magnesium oxides present in the reduced material is removed by treatment with an excess of dilute nitric acid having a specific gravity of about l .l2 to about 1.26. Such acid will not attack chromium-aluminum alloys having as little as 16 percent chromium by weight, but will readily dissolve metallic magnesium as well as magnesium oxide. Crushing the alloy to a fine powder helps the acid dissolve all the magnesium rapidly. It is not essential to remove any magnesium oxide present in the reduced mixture inasmuch as this compound is essentially inert during a coating operation and does not tend to sinter or adhere to the work pieces being coated or to the other ingredients of the coating pack. Where the hot magnesothermic reaction mixture has its vapor flushed out at high temperatures to flush out the relatively volatile magnesium metal remaining after the reduction is completed, the crude reaction produce can after crushing be directly used for diffusion coating. Where nitric acid washing is carried out, the washed material is rinsed with water, preferably to neutrality, filtered and dried before use.

Magnesothermic reduction can also be used in the same way to directly produce chromiumsilicon, chromium-aluminum-silicon, chromiumaluminum iron, molybdenum-silicon and tungsten-silicon alloys in the extremely finely divided form so highly desirable for diffusion coating work pieces. Silica makes a convenient source of silicon for such purposes and can be directly substituted for or added to the mixture being reduced without materially changing the reduction rate or temperature. The finely divided alloys can also be produced by magnesothermically reducing chromium, iron, molybdenum or tungsten oxides or other compounds of these metals in the presence of aluminum and/or silicon in elemental form. During such reduction the aluminum and/or silicon alloys with the metallic chromium, iron, molybdenum and tungsten as it is formed.

EXAMPLE Ill The setup described in connection with FIGS. 2 and 3 was prepared with a coating pack 50 as described in Example I, the supporting pack 84 being entirely calcinedalumina, and the vane beipg Wi-52 alloy. The coating heat was conducted at l,975F for 16 hours after which the vanes showed a uniform coating of about 0.0015 inch thickness throughout the surfaces in contact with the coating pack, with a slight throw of the coating about 1/16 to Vs inch further. This throw can be kept from the machined areas for example, by using the above-mentioned masking compositions in a layer about A inch thick or thicker against the portions of the work piece surface adjacent the coating pack and extending as far from it as the coating tends to otherwise throw.

EXAMPLE lV Turbine wheels as illustrated in FIG. 4, having an overall diameter of 9 A inches, a body diameter of 7 141 inches and having an air-foil cup width of inch were assembled with masking and coating mixtures as in that figure. The wheels were of MarM 246 alloy containing by weight 9% Cr, 10% Co, 1.5% Ti, 5.5% A1, 2.5% Mo, 0.15% Fe, 10% W, 1.5% Ta, the balance being nickel. The coating mixture was the same as used in Example I, the masking mixture like that of Example I except that the Ni-Al alloy corresponded to Ni Al, and the coating heat was 7 hours at 1,890F. The case depth produced was 0.0025 inch, with no coating of the masked areas.

It is preferred for the masking material to be in finely divided form such as powder particles no greater than about 30 mesh and better still smaller than 40 mesh, since they then present a large surface area to the coating atmosphere. However larger particle sizes can be used.

The masking of the present invention is also effective when the diffusion coating is carried out with a pack that is entirely inert, but through which coating vapors are passed, as disclosed in U.S. Pat. No. 3,286,684

granted Nov. 22, 1966. In fact the entire coating pack can be eliminated, and vapors alone used, particularly where the masking pack is adequate to support the work pieces. Correspondingly where localized coating is effected as in FIG. 2 and the work pieces can be adequately supported by the local coating pack, the remainder of the support can be omitted. However the coating pack tends to throw further when the inert support pack is not used, as compared to when it is used.

The masking of the present invention can also be effected by adding the masking composition as a slurry to a container in which the work piece is supported. The slurry can be an aqueous suspension, and the water content permitted to drain off so that the solids of the slurry remain as a pack in which the work piece remains embedded. The draining of the water is simplifled by providing the container with a large number of small perforations that do not permit much of the slurry solids to pass through.

The slurry can also be poured into a container in which the work piece is already supported by an inert pack of coarse alumina, 100 mesh for example. By having the slurry solids in the form of very fine powder, the slurry will be absorbed by the coarse pack and convert it to a very efficient mask.

The slurry can also contain thickening or setting agents such as gum tragacanth and bentonite, so that the work piece is more effectively supported by the slurry with or without the inert pack. One composition of this type has by weight 69 percent of a 325 mesh powder half A1 0 and half Ni Al, and 31 percent of a solution of 0.7 percent gum tragacanth in 1.7 percent ethanol and 97.6 percent water. This composition is a paste into which a work piece can be forced so that it stays in place while the composition is dried and is continued to be held in place during a firing and a diffusion coating heat. Thickening and setting agents so used should be decomposible by such high temperatures, or inert to the diffusion treatment.

The masking of the present invention is also helpful in the diffusion treatment of superalloy substrates to which a layer of diffusion coating material is adhered as in U.S. Pat. No. 3,312,546 granted Apr. 4, 1967. Such a layer of powdered aluminum-chromium-iron alloy in lacquer for example, is removed from, or not applied to, the areas to be masked and the masking composition applied there instead. When a halide-free atmosphere is used during the diffusion, no masking composition need be applied to the areas in which the diffusing alloy is not present.

The foregoing coating techniques are equally effective for unrestricted or masked coating of nickel-, cobalt-, or iron-base alloys suitable for use under stress at temperatures of 1,600F or higher. Such alloys are generally called superalloys and include those in which the principal ingredient or base is a mixture of any one, two or all three of the metals nickel, cobalt and iron. An example of the latter is an alloy containing equal parts by weight of nickel, cobalt and iron. The base content of the superalloys can vary from as much as percent or even 98 percent if thoria-dispersed nickel is considered an alloy, to as little as about 50 percent. A list of typical superalloys is included in French Pat. No. 1,490,744. Even ordinary stainless steels such as one containing 18% Cr and 8 percent nickel are effectively provided with good aluminum-rich coatings pursuant to the present invention, and will then show great resistance to oxidation at 1,900F, although such alloys are not suitable for use under stress at that temperature.

The preferred coatings of the present invention are diffused from alloys of aluminum and chromium in which the chromium content by weight is four to five times the aluminum content. Also the coating pack as well as the masking pack should contain from about A to about of an inert diluent like alumina that does not tend to sinter to the work pieces and also keeps the remainder of the pack ingredients from sintering to the work pieces. In addition to the alumina and magnesia referred to above, Cr O Zr0 and TiO can be so used. Diluents that tend to be chemically reduced by or react with hydrogen or pack ingredients at the coating temperatures, are not desirable.

The circulation of hydrogen shown in connection with FIG. 1 is not essential since argon or any other protective gas can be used in its place, although hydrogen seems to give somewhat better coatings and it is more easily monitored because of its combustibility. Indeed it is not necessary to circulate any gas around the inner retort, although hydrogen may then tend to accumulate in the space between the retorts. Such accumulation may form an explosive mixture with air, if a flushing of the gas in the inter-retort space is not performed. The added expense of using argon may be desirable where hydrogen tends to remain occluded or dissolved in the workpiece. To this end the diffusion coating can still be effected with hydrogen circulation, but in the cooldown step argon can be substituted for the hydrogen.

It is necessary to activate the coating pack to keep the coating temperatures and times within reason. Ammonium halides and particularly the chloride, are very effective additions to the pack for this purpose, and they need only be used in an amount from about 0.1 percent to about 1 percent, preferably 0.2 to 0.6 percent of the pack by weight. Ammonium fluoride, bromide and iodide are more expensive and more difficult to handle, and so are not preferred, although ammonium bromide works better than the fluoride or iodide. Other halides such as aluminum chloride, chromic chloride, chromous chloride and even hydrogen halides or the halogens themselves can be used, although those activators that are gaseous are more difficult to load in the inner retort. The activator need not be loaded at the time the work is, however, but can be added after the inner retort is closed, as by providing a conduit in the lid of that retort and introducing gaseous or liquid activator through the conduit after the work has been brought to coating temperature, for example. As pointed out above, such a fitted conduit can also be used with solid activators that are pre-mixed with the pack, and it will then simplify the flushing out of air from the pack as the activator vaporizes.

The use of a cover over the inner retort reduces the amount of activator needed by limiting the amount that escapes during the heat. Where larger losses can be tolerated the cover can be omitted, particularly if the flushing is restricted or completely eliminated. That retort can be alternatively fitted with a liquid-sealed lid as described for example in U.S. Pat. No. 2,844,273 granted July 22, 1958. The outer retort can also be omitted.

The nickel-aluminide masking of the present invention works well with all of the above-mentioned superalloys. Cobalt aluminide does not do a good job of masking nickel-based superalloys or superalloys containing as much as 25 percent nickel, although it is quite effective for masking cobaltand iron-based super-alloys. The masking of thoria-dispersed nickel which is strictly speaking not an alloy is not a problem inasmuch as it is not adversely affected when the active masking material is metallic nickel for example.

On the other hand where the substrate being diffusion coated has a chromium content of 1 percent or more, the masking of the present invention is further improved by the addition of a little chromium to the masking pack. Thus a masking pack composed of, by weight, 42.6 percent nickel and 6.5 percent aluminum powders pre-fired to form Ni Al, plus 1.6 percent chromium and 49.4 percent alumina powders that are not pre-fired, gives excellent masking of B 1,900 articles and has even less effect on the masked sections of the articles than a corresponding masking pack containing no chromium. The chromium addition to the masking pack makes a similar improvement when it is pre-fired with the nickel and aluminum.

When about 3 percent or more of chromium by weight is present in the masking pack it begins to cause the deposition of a chromium-containing coating on the masked sections of the articles under treatment, and is thus undesirable where dimensions are critical and no perceptible coating can be tolerated. As little as Apercent chromium in the masking mixture provides a noticeable effect in reducing the magnitude of the slight surface change generally brought about by chromium-free masking mixtures on superalloys containing 1 percent to 26 percent or more chromium. Such surface change is evidenced by a slight loss of fatigue strength and sometimes by intergranular attack, and is believed to be caused by a slight loss of chrmium content or perhaps some different alteration in the masked surface.

The tecnhique described in the foregoing Example II will, for instance, cause some weakening at the vane surfaces there masked. However the masking can be confined to the edge face of the opening leading to the vane interior and to this surface is then welded a cooling tube so that it is thus strengthened by the weld. This strengthening more than makes up for the weakening.

The best masking on nickel-, cobalt-, and iron-base alloys containing 1 percent or more chromium is with a masking mixture containing about 1.6% chromium, 25 to 66 2/3 percent of alumina, magnesia, Cr O or other inert diluent, the balance being a nickel aluminide ranging from Ni Al to Ni Al However a chromium content range of from 1.5 to 2.0% is very effective.

The chromium-containing masking compositions can be used in the same manner as the chromium-free ones. Also the chromium-containing masking composition can be pre-fired so that its chromium is diffused into the nickel or cobalt aluminide of the composition.

For commercial operation the masking combination of chromium and nickel or cobalt aluminide can be prepared as an article of commerce that can be sold, the purchaser then mixing it with the inert diluent. For this purpose there can be as little as 76 percent but as much as 9 percent, preferably 1 to 6 percent by weight chromium in such a mixture before the inert diluent is added. An optimum mixture has 3 to 3 k percent chromium, the balance being Ni Al, and such mixture is then diluted with an amount of alumina equal in weight to that of the chromium-Ni Al mixture.

Tungsten and molybdenum silicides whether or not prepared by the magnesothermic technique described above, are conveniently pelleted with a powdered glass such as a mixture of 78.2% SiO 20.6% B 0 and 1.2% A1 0 and the pellets used in a diffusion coating mixture, or the powdered silicides can be directly applied, electrophoretically for example, to tungsten or molybdenum bodies and then given a corresponding high temperature treatment to anchor the coating and provide a very effective diffusion-adhered protective layer.

A very satisfactory iron-chromium-aluminum coating alloy pursuant to the present invention was prepared by magnesothermic reduction. 1,000 grams of magnesium was melted in a steel retort under argon. After the meltdown and at room temperature 200 grams of Cr O powder, 200 grams Fe O powder and 50 grams A1 0 powder was placed on top of the solidified magnesium melt. The retort was covered with a semi-tight steel lid and this assembly placed in an lnconel outer retort. A low flow of argon gas, utilizing a flow rate of 6 c.f.h. was passed through the Inconel retort thereby blanketing the internal steel retort containing the magnesium and mixed oxides. The lnconel retort was then heated to a temperature of 1,800F and held at this temperature for 9 hours. After this time period the retort was cooled to room temperature in the argon atmosphere. The product was removed from the retort, crushed, and reacted with a solution of nitric acid containing one part by volume of 67 percent commercial nitric acid and one part by volume of water. Enough nitric acid was utilized to be 25 percent in excess of that required to combine with all the magnesium originally present. Since the action of nitric acid on the reacted powder was quite exothermic, it is helpful to have a water cooling jacket on the reaction kettle. Upon completion of the leaching process the resulting alloy powder was washed thoroughly to a neutral pH, vacuum filtered using a Biichner funnel and chamois filter pad, dried in an oven at 350F for four hours and pulverized. The powder contained 44% Cr, 47% Fe and 9% Al. Another ill such alloy contained 25% Cr, Al, the balance Fe, and was also a good diffusion coating source. Any such alloy containing 3 to Al and 12 to 60% Cr, the balance Fe, gives very good coatings on superalloys.

The addition of about 0.2% yttrium to the ironchromium-aluminum alloys further improves them.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. it is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In a process of diffusion coating with aluminum of a high temperature superalloy of cobalt or iron base, the step comprising preventing the aluminum coating from diffusing into undesired locations by covering said superalloy in selected areas with a masking composition which is a mixture of particles of an inert diluent and particles of an aluminide of nickel or cobalt, the aluminide having between one-third and three-fourths atoms of aluminum for every atom of nickel or cobalt, the diluent being from about one-fourth to about twothirds of the masking composition by weight.

2. In a process of diffusion coating with aluminum of a high temperature superalloy of nickel, cobalt or iron base, the step comprising preventing the aluminum coating from diffusing into undesired locations by cov ering said superalloy in selected areas with a masking composition which is a mixture of particles of an inert diluent and particles of an aluminide of nickel, the aluminide having between one-third and three-fourths atoms of aluminum for every atom of nickel, the diluent being from about one-fourth to about two-thirds of the masking composition by weight.

3. A process for diffusion coating selected portions of a high temperature cobalt or iron base superalloy with an aluminum-chromium mixture, wherein the improvement comprises the steps of providing a finely divided coating alloy of chromium with aluminum, placing said superalloy in a cementation pack with selected portions embedded in a mixture of said coating alloy and an inert diluent material to keep said mixture from sintering to the superalloy, the remaining portions of said superalloy being held in contact with a masking mixture comprising particles of an inert diluent and particles of an aluminide of cobalt or nickel, the aluminide having between one-third and three-fourths atoms of aluminum for every atom of cobalt or nickel, the diluent being from about one-fourth to about two-thirds of the masking mixture by weight, then heating the assembly to diffusion-coating temperature and activating the coating mixture with a halogen-containing vapor.

4. A process for diffusion coating selected portions of a high temperature nickel, cobalt or iron base superalloy with an aluminum-chromium mixture, wherein the improvement comprises the steps of providing a coating alloy of finely divided chromium with aluminum, placing said superalloy in a cementation pack with selected portions embedded in a mixture of said coating alloy and an inert diluent material to keep said mixture from sintering to the superalloy, the remaining portions of said superalloy being held in contact with a masking mixture comprising particles of an inert diluent and particles of an aluminide of nickel, the aluminide having between one-third and three-fourths atoms of aluminum for every atom of nickel, the diluent being from about onefourth to about two-thirds of the masking mixture by weight, then heating the assembly to diffusion-coating temperature and activating the coating mixture with a halogen-containing vapor.

5. A diffusion coating process for diffusing aluminum into the surface of a high temperature nickel, cobalt or iron base superalloy embedded in a coating pack, wherein the improvement comprises embedding at least a portion of the workpiece in a masking composition to prevent aluminum coating from diffusing into the surface of the masked portion of the workpiece, said masking composition comprising a mixture of inert particles, chromium, and nickel aluminide particles having between one-third and three-fourths atoms of aluminum for every atom of nickel, the inert particles being from about one-fourth to about two-thirds of the mixture by weight, the chromium content being from about to about 3 percent of the mixture by weight.

6. A diffusion coating process for diffusing aluminum into the surface of a high temperature cobalt or iron base superalloy embedded in a coating pack, wherein the improvement comprises embedding at least a portion of the workpiece in a masking composition to prevent aluminum coating from diffusing into the surface of the masked portion of the workpiece, said masking composition comprising a mixture of inert particles, chromium, and nickel or cobalt aluminide particles having between one-third and three-fourths atoms of aluminum for every atom of nickel or cobalt, the inert particles being from about one-fourth to about twothirds of the mixture by weight, the chromium content being from about A to about 3 percent of the mixture by weight.

7. The combination of claim 1 in which the aluminum is diffusion coated along with another metal.

8. The combination of claim 1 in which the aluminum is diffusion coated along with chromium.

9. The combination of claim 2 in which the aluminum is diffusion coated along with chromium.

10. The combination of claim 2 in which the aluminum is diffusion coated along with another metal.

11. The combination of claim 1 in which the diluent is alumina.

12. The combination of claim 2 in which the diluent is alumina.

13. In the process of diffusion coating a small portion of a metal work piece embedded in a diffusion coating pack containing the coating material and in an atmosphere containing a diffusion coating activator, the improvement according to which the diffusion coating pack is placed in contact only with the small portion of the work piece, and the remainder of the embedded workpiece surface is in contact with a pack that does not contain the coating material, and said coating pack is located within the pack that does not contain the coating material.

M. A mixture of chromium with an aluminide of nickel or cobalt, the aluminide having between one third and three-fourths atom of aluminum for every atom of cobalt or nickel, and the chromium content of the mixture being between about rt: and 9 percent by weight.

15. The combination of claim 14 in which the aluminide is Ni Al and the chromium content is between 3 and 3 r percent by weight.

16. A particulate mixture of chromium with an aluminide of nickel or cobalt and an inert diffusion coating diluent, the aluminide having between one-third and percent by weight. three-fourths atom of aluminum for every atom of co- 17. The combination of claim 16 in which the alumibalt or nickel, and the chromium content of the mixnide is Ni Al and the chromium content is between 1.5

ture being between about and 3 percent by weight, and 2.0 percent by weight. and the diluent content being between about A and 

2. In a process of diffusion coating with aluminum of a high temperature superalloy of nickel, cobalt or iron base, the step comprising preventing the aluminum coating from diffusing into undesired locations by covering said superalloy in selected areas with a masking composition which is a mixture of particles of an inert diluent and particles of an aluminide of nickel, the aluminide having between one-third and three-fourths atoms of aluminum for every atom of nickel, the diluent being from about one-fourth to about two-thirds of the masking composition by weight.
 3. A process for diffusion coating selected portions of a high temperature cobalt or iron base superalloy with an aluminum-chromium mixture, wherein the improvement comprises the steps of providing a finely divided coating alloy of chromium with aluminum, placing said superalloy in a cementation pack with selected portions embedded in a mixture of said coating alloy and an inert diluent material to keep said mixture from sintering to the superalloy, the remaining portions of said superalloy being held in contact with a masking mixture comprising particles of an inert diluent and particles of an aluminide of cobalt or nickel, the aluminide having between one-third and three-fourths atoms of aluminum for every atom of cobalt or nickel, the diluent being from about one-fourth to about two-thirds of the masking mixture by weight, then heating the assembly to diffusion-coating temperature and activating the coating mixture with a halogen-containing vapor.
 4. A process for diffusion coating selected portions of a high temperature nickel, cobalt or iron base superalloy with an aluminum-chromium mixture, wherein the improvement comprises the steps of providing a coating alloy of finely divided chromium with aluminum, placing said superalloy in a cementation pack with seleCted portions embedded in a mixture of said coating alloy and an inert diluent material to keep said mixture from sintering to the superalloy, the remaining portions of said superalloy being held in contact with a masking mixture comprising particles of an inert diluent and particles of an aluminide of nickel, the aluminide having between one-third and three-fourths atoms of aluminum for every atom of nickel, the diluent being from about one-fourth to about two-thirds of the masking mixture by weight, then heating the assembly to diffusion-coating temperature and activating the coating mixture with a halogen-containing vapor.
 5. A diffusion coating process for diffusing aluminum into the surface of a high temperature nickel, cobalt or iron base superalloy embedded in a coating pack, wherein the improvement comprises embedding at least a portion of the workpiece in a masking composition to prevent aluminum coating from diffusing into the surface of the masked portion of the workpiece, said masking composition comprising a mixture of inert particles, chromium, and nickel aluminide particles having between one-third and three-fourths atoms of aluminum for every atom of nickel, the inert particles being from about one-fourth to about two-thirds of the mixture by weight, the chromium content being from about 1/4 to about 3 percent of the mixture by weight.
 6. A diffusion coating process for diffusing aluminum into the surface of a high temperature cobalt or iron base superalloy embedded in a coating pack, wherein the improvement comprises embedding at least a portion of the workpiece in a masking composition to prevent aluminum coating from diffusing into the surface of the masked portion of the workpiece, said masking composition comprising a mixture of inert particles, chromium, and nickel or cobalt aluminide particles having between one-third and three-fourths atoms of aluminum for every atom of nickel or cobalt, the inert particles being from about one-fourth to about two-thirds of the mixture by weight, the chromium content being from about 1/4 to about 3 percent of the mixture by weight.
 7. The combination of claim 1 in which the aluminum is diffusion coated along with another metal.
 8. Th combination of claim 1 in which the aluminum is diffusion coated along with chromium.
 9. The combination of claim 2 in which the aluminum is diffusion coated along with chromium.
 10. The combination of claim 2 in which the aluminum is diffusion coated along with another metal.
 11. The combination of claim 1 in which the diluent is alumina.
 12. The combination of claim 2 in which the diluent is alumina.
 13. In the process of diffusion coating a small portion of a metal work piece embedded in a diffusion coating pack containing the coating material and in an atmosphere containing a diffusion coating activator, the improvement according to which the diffusion coating pack is placed in contact only with the small portion of the work piece, and the remainder of the embedded workpiece surface is in contact with a pack that does not contain the coating material, and said coating pack is located within the pack that does not contain the coating material.
 14. A mixture of chromium with an aluminide of nickel or cobalt, the aluminide having between one-third and three-fourths atom of aluminum for every atom of cobalt or nickel, and the chromium content of the mixture being between about 1/3 and 9 percent by weight.
 15. The combination of claim 14 in which the aluminide is Ni3Al and the chromium content is between 3 and 3 1/2 percent by weight.
 16. A particulate mixture of chromium with an aluminide of nickel or cobalt and an inert diffusion coating diluent, the aluminide having between one-third and three-fourths atom of aluminum for every atom of cobalt or nickel, and the chromium content of the mixture being between about 1/4 and 3 percent by weight, and the diluent content being between about 1/4 AND 2/3 percent by weight.
 17. The combination of claim 16 in which the aluminide is Ni3Al and the chromium content is between 1.5 and 2.0 percent by weight. 